Separation of cobalt and nickel was investigated using a 4-stage countercurrent mixer-settler cascade, employing 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester as the extractant. Extraction equilibrium formulations were established up to high loading ratios and extraction constants were determined, based on a batch extraction system in both single metal and binary metal systems. The separation of cobalt and nickel was then carried out using the 4-stage countercurrent mixer-settler cascade. Selective extraction of cobalt was achieved by the extraction step, while a small amount of nickel was also extracted. Nickel in the organic solution was removed by scrubbing with the solution containing the metals, while the recovery of cobalt was decreased. The concentration profiles of the metals with the countercurrent mixer-settler cascade were calculated based on the determined extraction equilibrium formulations and the material balance, although the calculation diverged when the pH was high especially at the 4th stage.